Testosterone is associated with two behaviors in males: aggressive dominance of other males, and sexual activity. E.g., male red deer live peacefully together for most of the year. But in October their testosterone increases five-fold. They fight for territory. Female red deer select land that has sufficient food for raising fawns. The males that control the best territories mate with the most females.[3]

Testosterone is an anti-depressant, in men and women.[4] It increases friendliness. It reduces anger, depression, fatigue, confusion, nervousness, and irritability.[5]

Testosterone is necessary for sexual activity. However, the amount of testosterone needed for sexual activity is low.[6] Testosterone injections don't increase sexual activity or sexual desire in young men.[7] Men have more than enough testosterone for sex.

Testosterone varies between men more than four times.[8] Football players have the most testosterone. Ministers have the least.[9]

Men and women with high testosterone commit more violent crimes. They're more unruly in prison. Parole boards judge them more harshly.[10]

Bachelors have more testosterone than married men. Childless husbands have more testosterone than fathers.[11] Low testosterone in men correlates with marital satisfaction. Low testosterone improves emotional expressiveness, parent-child communication, and "androgynous behaviors"-the ability to use feminine skills when necessary.[12] A number of studies have shown that testosterone levels are affected not only by medical factors, but by meteorological, conjugal, social, and other factors as well. Men's testosterone is higher during some parts of the year than others. In most parts of the continental United States, the highest levels usually occur in August and September. Warm moist air at night tends to raise testosterone, and cold mornings tend to decrease it. Men's hormone levels are also influenced by their partners. One study (Toronto, Ontario 2006) found that increases or decreases in men's levels of testosterone are often correlated to their wives' menstrual cycles: the husband's testosterone tends to increase when the wife is ovulating, and decrease again afterward. This effect was observed particularly in younger couples.

Female executives and attorneys have more testosterone than secretaries, teachers, nurses, and housewives.[16]

Women who don't use condoms are less depressed than women who use condoms, or who aren't sexually active. Women who do not use condoms become increasingly depressed as time elapses since their last sexual encounter. They also seek new partners sooner after ending a relationship. Women apparently absorb an anti-depressant hormone, possibly testosterone, from semen.[17]

Winning boosts testosterone. Losing reduces testosterone. Find a sport you can win at, e.g., softball vs. baseball. You have to win at something physical and indicative of social dominance, e.g., winning the lottery doesn't affect testosterone.[22]

The presence of females, especially new females, boosts male testosterone.[23] Speculatively, young women increase men's testosterone more than older women. You don't need to have sex to get this effect. The effect is produced via pheromones in women's sweat. Play a sport you can win, on a team with sweating young women.

Pre-pubescent boys have little testosterone. After puberty, testosterone increases in teenagers. Testosterone is generally stable until men are in their 60s. Testosterone then decreases in old age.[24] Diminishing testosterone doesn't sufficiently explain the declining sexual activity associated with aging.[25]

A study in the United States, Congo, Nepal, and Paraguay found that Americans have the highest testosterone when young, and the least in old age, but testosterone levels are nearly constant from adolescence to old age in traditional societies in the other three countries.[26] This difference may be due to how American men and women interact. North American young men interact with many young women, in high school and college. Older American men, however, have interaction with far fewer non-related young women. In rural societies in many other countries, by contrast, men generally interact with roughly the same number of young women throughout their adult lives.

Women's ovaries produce estrogen and progesterone.[27] Estrogen increases in girls at puberty, making them develop breasts and hips. Estrogen drops at menopause (around age 50).

Estrogen is associated with ovulation and sexual receptivity. In female animals, estrogen increases male interest in the female,[28] female solicitations of males for sex,[29] and male sexual performance.[30]

For the first 10-12 days after menstrual bleeding ceases, estrogen and testosterone are low but increasing. Progesterone is very low. A woman described this time as "confident and social, on the prowl."[43]

Women ovulate around days 13-15. Estrogen and testosterone peak. Progesterone remains low. Brain scans show that women respond more dramatically to pictures of nude men during ovulation.[44] Sexual behavior peaks. Ovulating women in singles bars wear, on average, more jewelry and makeup than they do at other times of the month, according to Australian research. They are more likely to initiate physical contact with men.[45]

During the two weeks after ovulation, progesterone dominates. Estrogen remains moderately high. Testosterone diminishes. Progesterone makes women not want sex, but feel nurturing. A woman described this time as "too tired for an orgasm, but could have touched all night."[46]

Progesterone and estrogen drop a few days before menstruation. This relates to premenstrual syndrome (PMS) in some women.[47]

Individuals who regularly cuddle can become addicted to each other. They can experience oxytocin withdrawal when apart[48].

Orgasm spikes oxytocin to five times normal levels. The refractory or disinterested period some people feel after sex may be due in part to oxytocin overdose (too much of any drug reduces sexual interest).[49][50]

Pheromones are chemicals in sweat and urine. Animals use pheromones to sense whether another animal is male or female, related or unrelated,[51] sexual receptiveness in females, and dominance in males. The vomeronasal organ (VNO) in the nose detects pheromones. The VNO is distinct from the nose's smell faculty-pheromones are a "sixth" sense. Many scientists believe that the VNO in humans merges into the sinus during fetal development, and that many people do not have a functional VNO.

The VNO connects to the accessory olfactory bulb, which is a brain area distinct from the main olfactory bulb (which processes our sense of smell). From there, pheromone information goes directly to the reptilian brain, and the limbic (old mammalian) brain area for maternal behavior. Pheromone information never gets to our cerebral cortex, the recently evolved brain area for consciousness and abstract thinking.[52] We have no conscious awareness of pheromones.

Male mice missing the pheromone receptor gene are unable to sense "maleness" in other mice. These male mice treat all other mice as female. They try to mate with male mice as well as with female mice. They don't fight other male mice.[54]

Parents with genetically different immune systems produce offspring with strong immune systems. Parents with genetically similar immune systems can be infertile or have miscarriages. Females sense a male's immune system via major histocompatibility complex (MHC) molecules. The VNO appears to sense MHC.

The sweat of men with genetically different MHC attracts women. The sweat of men with genetically similar MHC repulses women.

Pregnant female mice seek males with genetically similar MHC. These males are likely to be relatives and will protect their baby relations.[55]

(Perhaps we're actually offended by pheromones, not body odor. What if I had to negotiate a business deal with a man who makes me feel weak and queasy? What if I had to work next to a woman who makes me unable to think about anything but sex? Perhaps we shower daily and use anti-perspirants so that we can get along with each other.)

Men's sex drive results from one hormone. Testosterone varies 50% or more daily,[57] varies even more between men, and diminishes with age, but, compared to women, the male sex drive is relatively constant. In general, mens' sex drives are higher.

In contrast, three hormones-estrogen, progesterone, and testosterone-control women's sex drive. Women's interest in sex varies in character and intensity over their menstrual cycles, during pregnancy and nursing, and during menopause. Women's sex drives can be as strong or stronger than men's, but only at specific times.

Female mammals will have sex only in certain places. E.g., female elephant seals select a beach with minimal danger from predators. Female red deer select a meadow with abundant food to support fawns. Many human females like familiar surroundings, moderate temperature, etc.

Female primates prefer to mate with specific males. Whether a male physically dominated other males (e.g., gorillas), or built a social network (e.g., baboons), or is one's life partner (e.g., gibbons), rejecting inappropriate males is central to female sexuality.

Women generally have better senses of smell, taste, and hearing. Men generally have better vision.[59]

Women often have better verbal skills. In general, they have better fine motor skills, e.g., threading a needle or connecting small wires. Men are often better at "directed motor skills," e.g., throwing a ball, and "visual-spatial abilities," e.g., map reading and mental rotation of 3-D objects.[60]

70% of men tend to use one brain hemisphere at a time while working on a task. Men can often concentrate on a task for long periods.[61] Focusing on an activity without distractions soothes men, e.g., driving a motorcycle across the country.

Most women multi-task, e.g., minding a baby while cooking dinner while talking on the telephone. Approximately 70% of women (as opposed to 35% of men) are adept at multitasking, according to one study. So-called "woman's intuition" (HSI, as it is more technically called) results from using her whole brain to solve a problem.

XX (female) foeti exposed to testosterone develop masculinized features, e.g., a clitoris as large as a small penis, and (sometimes) masculine thinking patterns and behaviors. Conversely, XY (male) foeti not exposed to testosterone become feminized.[62]

Switching the hormones of baby animals-rats, ferrets, pigs, finches, and monkeys[63] - makes males grow up to behave as females, and vice versa.[64] Females attempt to mount females. Males assume the lordosis female sexual posture to get males to mount them.[65] Given a choice of male or female sexual partners, the hormonally manipulated animals choose their own sex.[66]

Some genetic disorders and hormone imbalances expose human XX (female) foeti to testosterone or estrogen, or prevent testosterone exposure of XY (male) foeti. These XY (male) adults look and act like women (sometimes including sexual attraction to men). The XX (female) girls prefer to play with trucks instead of dolls.[68] The adult women have masculine features, behavior, and sexual attraction to women.[69]

Gender identity, sexual biology, and cognitive skills appear to develop at different stages of pregnancy. Abnormal fetal hormones at certain points in pregnancy can produce one gender abnormality, while the individual otherwise develops normally. E.g., a man could think of himself as masculine, wear men's clothing, etc., but feel sexual attraction to other men.[70]

↑O'Connell, Sanjida. "Sniffing out a partner could lead to good health for your children," The Guardian, September 27, 2001, http://www.guardian.co.uk/ Archive/Article/0,4273,4264733,00.html. Claus Wedekind of the University of Bern, Switzerland studied women's attraction to men's sweat. Carole Ober of the University of Chicago studied the effect of oral contraceptives.

↑C.D. Toran-Allerand, "On the Genesis of Sexual Differentiation of the Central Nervous System: Morpho-Genetic Consequences of Steroidal Exposure and Possible Role of Alpha-Fetoprotein," in G.J. Devries et al. (eds), Sex Differences in the Brain: Special Issue of Progress in Brain Research 61 (1984): 63-98.